As a method for forming an image according to the thermal transfer, a sublimation type thermal transfer recording method is known, wherein a thermal transfer sheet in which a colorant layer which includes sublimation type dye is provided on a substrate, and a thermal transfer image-receiving sheet in which a receiving layer is provided on a supporting member are superposed to form a full color image. Since this method uses the sublimation type dye as a color material, it is possible to adjust freely the density and the gradation dot unit by dot unit, a full-color image can be clearly expressed as the original image on the image-receiving sheet. Therefore, this method has been applied in the color image formation for digital cameras, video recorders, computers or the like. Its image can rival silver halide photography with high quality.
In order to obtain good printing properties on the thermal transfer image-receiving sheet to be used for the sublimation transfer recording method, it is known that it is preferable to give heat resistance and cushioning to the thermal transfer image-receiving sheet. With regard to the functional requirements for the heat resistance and the cushioning, for instance, in Patent Literature 1, a thermal transfer image-receiving sheet in which a thermal insulation layer containing a hollow polymer is provided between the supporting member and the receiving layer has been proposed. Further, in addition to the thermal insulation layer as described above, some layers referred to as “micro-voids layer”, “hollow particles layer”, “void film layer”, “void PP layer”, etc., are known as the layer which imparts the heat resistance and the cushioning and is provided between the supporting receiving layer and the receiving layer.
Incidentally, with regard to the thermal transfer image-receiving sheet including the thermal insulation layer, thermal history during manufacturing of the thermal transfer image-receiving sheet generates shrinkage stress in the thermal insulation layer, and thus, a curl owing to the shrinkage stress is liable to occur in the thermal insulation layer. Furthermore, thermal history during image formation in which a thermally transferred image is formed on the receiving layer also generates shrinkage stress in the thermal insulation layer, and thus, a curl is liable to occur on the entire printed matter which has the thermally transferred image formed on the receiving layer thereof. Due to a relationship between the curl and the supporting member of the thermal transfer image-receiving sheet, in many cases, the curl is generally curled inwardly with respect to the thermal transfer image formed face. Thus, it caused a decrease in the appearance of the printed matter which was obtained by forming a thermal transfer image onto the receiving sheet of the thermal transfer image-receiving sheet, for example, when displaying photograph as the printed matter to the wall. Further, even when the curling does not immediately occur on the thermal transfer image formation, the curling may occur in time course if shrinkage stress is inherent in the thermal insulation layer. In addition to the layer referred as to “thermal insulation layer”, this point is similarly applicable to various layers which each are provided between the supporting member and the receiving layer for the purpose of heat resistance and cushioning properties.
In order to reduce the occurrence of the curling, it is considered that it is an effective countermeasure to reduce the shrinkage stress generated in the thermal insulation layer, and, as one means of reduction measures of the shrinkage stress, to reduce the thickness of the thermal insulation layer can be mentioned. However, in a typical thermal insulation layer, when reducing the thickness of the thermal insulation layer to be able to reduce the curling effectively, the heat resistance and cushioning properties which are functions required intrinsically in the thermal insulation layer are also reduced, and thus, it becomes difficult to form an image having a sufficient density on the receiving layer. In other words, it is difficult to form a high quality image on the receiving layer. Furthermore, in many cases, the supporting member of the thermal transfer image-receiving sheet happens to include foreign matters, defects or the like. If the thinned thermal insulation layer is provided on the supporting member having such defects or the like, it is impossible to hide such defects or the like of the supporting member. Furthermore, depending on the characteristics of defects or the like of the supporting member, it may occur density unevenness in the thermal transfer image formed on the receiving layer, which is followed by causing a reduction in printing quality.
Under these circumstances, in Patent Literature 2, as a supporting member for using in a printing paper for digital photographic printer suitable to obtain an image of high print quality, a biaxially oriented layered polypropylene film in which a front face side layer and a rear face side layer are provided on both side of a substrate layer which is made of polypropylene-based polymer composition containing specified amounts of calcium carbonate and titanium oxide. According to Patent Literature 2, by adopting the biaxially oriented layered polypropylene film as the supporting member of the thermal transfer image-receiving sheet, it is possible to obtain a high-quality image. However, with respect to the biaxially oriented layered polypropylene film has been proposed in the literature, when the thickness of the supporting member is reduced in order to suppress the occurrence of curling, it becomes impossible to obtain a high quality image. That is, it is not possible to achieve both of the suppression of occurrence of curling, and the obtaining of a high-quality image.
Further, in Patent Literature 3, a thermal transfer image-receiving sheet has been proposed, wherein the thermal transfer image-receiving sheet comprises a substrate sheet, and a porous layer and a receiving layer which are provided on one surface of the substrate sheet in this order, and wherein the porous layer is composed of a porous film containing polypropylene resin and has a thickness of not more than 30 μm. According to the thermal transfer image-receiving sheet which has been proposed in Patent Literature 3, it is said that it is possible to reduce changing amount of the curl, and to improve the quality after storage under various environments. However, there is room for improvement from the viewpoint of the print quality.
In other words, in the case that the conventionally known supporting members, or the conventional layers are adopted as supporting member of the thermal transfer image-receiving sheet, or a layer provided between the supporting member and the receiving layer, although it is possible to suppress the occurrence of curling when decreasing the thickness of these layers, it becomes difficult to satisfy the printing quality.